The present invention relates to epitaxial reactors and, more particularly, to a gas collector for epitaxial reactors.
Continuing advances in the semiconductor industry have resulted the development of highly complex thin-film deposition processes for fabricating semiconductor devices that are packaged for use in the manufacture of sophisticated electronic devices. The thin films of material that are deposited on semiconductor wafers are often referred to as epitaxial layers. High speed electronic transistors, quantum-well diode lasers, light-emitting diodes, photodetectors, and optical modulators incorporate structures composed of numerous epitaxial layers ranging in thickness from several microns to as thin as a few tenths of a nanometer. These epitaxial layers are typically deposited, or grown, on a single-crystal substrate, i.e., the semiconductor wafer.
One method of forming epitaxial layers on a semiconductor wafer is known as chemical vapor deposition (CVD). In a typical manufacturing process of a wafer, for example, silicon or gallium arsenide in extremely pure crystalline form is overlaid sequentially with numerous layers of materials which function as conductors, semiconductors, or insulators. Each subsequent layer is ordered and patterned such that the sequence of layers forms a complex array of electronic circuitry. The semiconductor wafer can then be subsequently cut along predetermined scribe lines into individual devices, commonly referred to as xe2x80x9cchips.xe2x80x9d These chips ultimately function as key components in electronic devices ranging from simple toys to complex supercomputers.
CVD processes normally take place within a reaction chamber. Initially, the semiconductor wafer is placed within a reaction chamber containing an inert atmosphere, and the temperature within the reaction chamber is elevated. Reaction gasses containing the compound or element to be deposited are then introduced to react with the surface of the semiconductor wafer, which results in deposition of the required film onto the semiconductor wafer. The reacted gasses are continually introduced and removed from the reaction chamber until a requisite film thickness has been achieved.
An example of an epitaxial reactor is described in U.S. Pat. No. 4,961,399, to Frijlink, which is incorporated herein by reference. This patent describes a reactor into which reaction gasses are introduced via a quartz funnel that is located at the center of the reactor. The reaction gasses then flow radially outward towards a quartz ring that bounds the reactor. Along the circumference of the quartz ring are equidistant slits, which collect the reacted gasses and increase the uniformity of the distribution of the reaction gasses within the reaction chamber. Bounding the upper portion of the reaction chamber is a quartz disk. The quartz disk seals against O-rings, witch are positioned behind the quartz ring. Because quartz is a brittle and inflexible material, the quartz disk does not seal against the quartz ring. Instead, a gap is provided between the quartz disk and the quartz ring to prevent chipping of either.
This gap between the quartz disk and the quartz ring can cause problems within the reactor. For example, reaction gasses can escape through the gap and can form deposits outside the reaction chamber. These deposits can interfere with the working of the reaction chamber and can also flake off and act as contaminants. Although a narrower gap can be provided, if a hard foreign body wider than the gap is introduced into the gap, such as during the opening of the reaction chamber, the foreign body could prevent the quartz disk from sealing properly over the reaction chamber or can cause chipping of either the quartz disk or the quartz ring.
An attempted solution to the above-mentioned problems is described in U.S. Pat. No. 4,976,217 to Frijlink, which is incorporated herein by reference. This patent describes a collecting crown or gas collector, which is both used to collect reaction gasses from the reaction chamber and also to provide a seal between the reaction chamber and a quartz disk or cover.
The gas collector and reaction chamber of the prior art is illustrated in FIGS. 1 and 2. The gas collector 1 is mounted on a supporting platform 4 by a horizontal plate 10 that rests upon the supporting platform 4. The supporting platform 4 is typically formed from quartz and is positioned within a cylindrical body 19 of the reactor that surrounds the reaction chamber and the gas collector 1. The cover 8 of the reaction chamber bounds the top of the reaction chamber and seals against the upper ridge 6 of the gas collector 1 and against toric joints 20 within the cylindrical body 19.
The gas collector 1 is further illustrated in FIG. 3. The gas collector 1 is formed from a folded plate of molybdenum having elastic properties. The molybdenum plate is folded along horizontal folding lines 13 and vertical folding lines 14 to form multiple flat plates 17, 5, 18, 9, 3, 10 that are connected to one another along the folding lines 13, 14. Also, two plates 2, 3 are touching without being fixed to each other. The combination of plates 17, 5, 18, 9, 3, 10 form a conduit 30 that encircles the reaction chamber. One of the plates 17 includes regularly spaced inlets holes 12 that collect the reaction gasses from the reaction chamber. Instead of the inlet hole 12, as shown below on the right-hand side of FIG. 3, the wall plate 17 can be provided with folded lower projections 15, which separate the movable lower edge 2 away from the fixed edge 3 to leave a slot between the edges 2, 3 through which the reaction gas can then pass.
The ""217 patent states that an essential element of the gas collector 1 is the vertical baffle plate, which is constituted by plates 17, 3 with the lower edge 2 of the upper plate 17 being pressed with a sliding motion against the upper edge of the lower plate 3. The horizontal plates 10 that are connected to the lower plates 3 serve to place the gas collector 1 on the edge of the platform 4 (best shown in FIG. 5). Furthermore, the top plate 5 is inclined and includes an upper ridge 6.
Due to the vertical folding lines 14, the conduit 30 is divided into successive parts. The whole of the gas collector 1 constitutes a polygon surrounding the platform (the partly shown polygon in FIG. 2 is a polygon having 24 sides). When the cover 8 is not yet positioned over the gas collector 1, the upper ridge 6 of the gas collector 1 is slightly higher than the upper surface of the cylindrical body 19 of the reactor. Thus, when the cover 8 is positioned on the gas collector 1, the cover 8 presses against the ridge 6 of the top plate 5. The downward force by the cover 8 also causes firm contact of the horizontal plate 10 with the top surface 31 of the platform 4. Because the horizontal plate 10 is fixed on the fixed platform 4, plates 3, 9, 18, 5 constitute a spring which allows the ridge 6 and upper plate 17 to be moved with respect to the fixed platform 4. The springing action of the plates 3, 9, 18, 5 causes the upper plate 17 to rise with respect to the lower plate 3 after the force against the top plate 5 by the cover 8 is removed.
As illustrated in FIG. 4, the gas collector 1 can be formed from a molybdenum plate made of a single cut piece. The folding lines 13, 14 are marked, for example, with a dotted line of holes made by of a laser. Consequently, during the manufacturing of the gas collector 1, the plate is folded along the folding lines 13, 14 provided. An exhaust 21 can also be provided and is connected to a tube 29 (best shown in FIG. 2) that exhausts the reaction gasses from the conduit formed by the gas collector 1.
The ""217 patent states that reaction gasses introduced into the reaction chamber cannot pass between the ridge 6 of the gas collector 1 and the cover 8, and reaction gasses cannot pass either between the horizontal plate 10 and the platform 4; and therefore, the reaction gasses pass exclusively through the inlets 12 and do not form dirtying deposits on either the cover 8 or on the periphery 11 of the platform 4. However, actual use of this gas collector 1 has proved otherwise.
The disclosed gas collector 1 suffers several problems. A non-exhaustive list of these problems include contamination of the periphery 11 of the platform, the top plate 5, and the cylindrical body 19; uneven gas flow and gas density of the reaction gasses through the reaction chamber; and contamination within the reaction chamber. Many of these problems stem from the gas collector 1 being formed a sheet of molybdenum, which is folded along folding lines 13, 14. Sheet metal structures are very difficult to manufacture to a high degree of dimensional precision. For example, the bending of the sheet metal along the folding lines 13, 14 is imprecise at best. Furthermore, the gas collector 1 is constructed using small screws and nuts, which do not lend themselves to maintaining a high degree of dimensional precision.
The gas collector 1 being formed by sheet metal, therefore, provides poor dimensional precision or tolerances for both the horizontal plate 10 extending over the platform 4; the positions of the inlets 12 in the upper plate 17; the connections of the upper plates 17 with one another; and the ridges 6 of the top plate 5. Another reason for the poor dimensional tolerances of the gas collector 1 results from thermal stressing of the sheet metal during the deposition process. As the thin molybdenum sheet metal of the gas collector 1 expands and contracts during each process cycle, the gas collector 1 eventually buckles and warps, thereby destroying the dimensional integrity of the gas collector 1.
The result of these poor dimensional tolerances is that gas collector 1, although purporting to seal the reaction gasses within the reaction chamber except through the inlets holes 12, provides numerous locations for the reaction gasses to escape the reaction chamber. For example, the ridge 6 often fails to complete seal the gas collector 1 against the cover 8. As such, reaction gasses are free to flow past the ridge 6 and form deposits, for example, on the top plate 5, rear plate 18, and on the cylindrical body 19.
The deposits formed on the gas collector 1 and cylindrical body 19 require frequent cleaning of both the gas collector 1 and the cylindrical body 19. For example, in one application, the disclosed gas collector 1 was being cleaned after approximately every 20 process cycles. Furthermore, because the gas collector 1 is formed by molybdenum sheet metal, the deposits on the gas collector 1 are very difficult to remove without damaging the gas collector 1. This limits the number of cleanings of a particular gas collector 1, on average, to three times before the gas collector 1 is replaced.
A disadvantage of having deposits on the gas collector 1 is that the deposits can flake off and contaminate the inside of the reactor. These flakes can interfere with the deposition process on the semiconductor wafers and can cause the subsequent rejection of the wafers. With the disclosed gas collector 1 of the prior art, for example in one application, approximately 13.5% of the wafers are rejected for contamination caused by flakes.
The flakes are caused, for example, because the gas collector 1 is formed from molybdenum sheet metal. Molybdenum is a material onto which deposits cannot firmly adhere. As such, these deposits can easily flake off when stressed. Flexing of the molybdenum sheet metal creates the stresses within the deposits that cause the formation of the flakes or chips. The sheet metal flexes for several reasons, one of which is that the gas collector is formed from sheet metal, and sheet metal is notorious for flexing, which also relates to why constructs made from sheet metal have poor positional tolerances. A second reason is that the gas collector 1 is designed to be flexed. As stated above, the plates 3, 9, 18, 5 constitute a spring; and therefore, any deposits formed on the plates 3, 9, 18, 5 are subject to stress during the opening and closing of the cover 8. Still another reason for flexing is that molybdenum expands and contracts because of the heating and cooling of the gas collector 1 during a process cycle.
Another source of flakes is that the horizontal plate 10 extends radially inward towards the reaction chamber from the gas collector 1. As illustrated in FIG. 5, in the gas collector 1 of the prior art, the horizontal plate 10 extends inward to about half the width of the top surface 31 of the platform 4 and leaves the other half of the top surface 31 exposed. During processing, deposits build up on both the horizontal plate 10 and the exposed half of the top surface 31. It has been noticed that movement of the horizontal plate 10, for example, when the reactor cover 8 is closed to compress the gas collector 1, causes the horizontal plate 10 to move relative to the platform 4. This movement of the horizontal plate 10 on the platform 4 stresses deposits that bridge the top surface 31 of the platform 4 and the horizontal plate 10 and disadvantageously cause the formation of flakes.
The reactor disclosed above in U.S. Pat. No. 4,961,299, with which the gas collector 1 of the prior art is used, is designed such that reaction gasses flow evenly from the center of the reaction chamber outward into the gas collector 1. A flow is considered even if the gas densities and velocities at a given radius away from the center of the reaction chamber are substantially equal. If the reaction gasses are not flowing evenly from the center of the reaction chamber, the deposition process varies depending upon the location of the wafers within the reaction chamber because the densities of the various constituents of the reaction gasses also vary. As such, the thickness and quality of the deposition can vary from one wafer to the next, even within the same batch process. For example, when depositing AlxGaAs using the gas collector 1 of the prior art, the percentage (x) of aluminum being deposited varies not only from one batch of wafers to the next, but also varies within wafers in single batch and also within a single wafer.
Obtaining an even flow of reaction gasses, however, is difficult with the gas collector 1 of the prior art. An even flow of reaction gasses results from the gas collector 1 providing an identical pressure differential between the reaction chamber and the conduit 18 inside the gas collector 1. As stated above, however, the gas collector 1 of the prior art is constructed with poor positional tolerances which provide gaps between the ridge 6 and the cover 8; gaps between adjacent front plates 17; and gaps between the horizontal plate 10 and the platform 4. Additionally, the holes used to form the bending lines 13, 14 also provide additional gaps in the gas collector 1. These gaps are not consistent along the circumference of the gas collector 10 and create different pressure differentials along the circumference, which therefore causes the reaction gasses to have different flow patterns depending upon the radial direction the reaction gasses flow.
Furthermore, the inlet holes 12 are positioned on a front plate 17 that is movable relative to the platform 4. This movement of the inlet holes 12 relative to the reaction chamber can change each time the cover 8 is raised and lowered and causes different flow rates that can vary during each batch process and/or from each gas collector 1. For example, the amount of pressure placed on the gas collector 1 when the cover 8 of the reactor is closed can vary, and this can cause the positions of the inlet holes 12 to vary. Also, for example, the positions of the inlet holes 12 can vary even if the pressure of the cover 8 remains the same because the flexibility of sheet metal forming the gas collector 1 varies over time. Furthermore, because the gas collector 1 of the prior art is made from sheet metal and is constructed used small screws, the flexibility or springiness of a particular gas collector 1 cannot be formed consistently, and therefore, the springiness varies from one gas collector 1 to the next. These positional variations of the inlet holes 12 cause the flow pattern of reaction gasses through the reaction chamber to change, and this change of the gas flow pattern affects the deposition process. Thus, the positioning of inlets 12 in a member movable relative to the reaction chamber causes an undesirable variance in the deposition process.
Another problem resulting from use of the gas collector 1 of the prior art is the creation of standing eddy currents adjacent the gas collector 1 that trap reaction gasses during processing. This can be a problem, for example, when a processing sequence using the reaction chamber changes reaction gas mixtures during the process. During the process of changing reaction gasses, the old reaction gasses are purged from the reaction chamber and the new reaction gasses are then introduced into the reaction chamber. However, because the old reaction gasses can be trapped in the standing eddy currents, these old reaction gasses can be subsequently reintroduced into the reaction chamber during the processing with the new reaction gasses, and this contamination of the new reaction gasses can have adverse effects on the process. The standing eddy currents are formed in sharp corners of the gas collector 1, such as illustrated in FIG. 5, where, for example, vertical plate 17 meets horizontal plate 10.
An example of this problem occurs during the doping of a GaAs semiconductor. Silicon is used for N-type doping of GaAs and zinc is used for P-type doping of GaAs. If the gasses that provide these dopants are not completely removed before the other gas is introduced, the active regions created by the doping can become washed out, which degrades the performance of the device being manufactured.
There is therefore a need for a gas collector that prevents the problems of the prior art, which include leakage of reaction gasses past the gas collector; flakes formed during the flexing of the gas collector; and uneven flow caused by the various gaps introduced into the gas collector.
This and other needs are met by embodiments of the present invention which provide a gas collector for collecting gasses from within a reaction chamber of a reactor. The gas collector includes a rigid body, in which is defined a conduit, inlets, an outlet, and a seal. The seal cooperates with a removable lid of the reactor to prevent escape of the gasses from the reaction chamber. Also, the inlets direct the gasses from the reaction chamber into the conduit, and the outlet exhausts the gasses from the conduit into an exhaust pipe of the reactor.
By using a rigid body, the body resists flexing, which creates stress on deposits formed on the body. The stressing of the deposits creates chips or flakes of the deposited material, which can cause the rejection of a device being coated within the reactor. The reduction of flexing by using a rigid body advantageously reduces incidences of chips and flakes.
In another embodiment of the present invention, a gas collector includes a body, in which is defined a conduit, inlets, an outlet, and a seal. The body also includes at least two members separate from one another. The body can also include inter-member seals that reduce the flow of the gasses across an interface between a first member and a second member of the body. The inter-member seals are disposed between the first member and the second member and allow movement of the first member relative to the second member. One of the inter-member seals can be formed on an outer wall, which is opposite an inner wall adjacent the reaction chamber. Also, each of the inter-member seals can allow movement of the first member relative to the second member in a common direction.
In one aspect of the gas collector, one of the first and second members defines a groove, and an other of the first and second members includes a projection with the projection engaging the groove to form the inter-member seal. Alternatively, the inter-member seal includes a male portion and a female portion interengaging with one another. Furthermore, the male portion is at least partially inserted into the female portion when the gas collector and the lid are separated, and the male portion extends about 40% to about 60% of the depth of the female portion when the gas collector is engaged with the lid.
By providing at least two member separate from one another, the conduit of the gas collector can be more easily cleaned by separating the two member before cleaning. In contrast, access to the conduit of the gas collector of the prior art is very difficult because the gas collector is formed by a single piece of sheet metal.
In yet another embodiment of the present invention, a gas collector includes a body, in which is defined a conduit, inlets, an outlet, and a seal. Also, the inlets are stationary relative to the reaction chamber. Furthermore, the body can include a first member stationary relative to the reaction chamber and a second member movable relative to the reaction chamber with the inlets disposed in the first member. By positioning the inlets on a member that is stationary relative to the reaction chamber, the flow pattern of the gasses through the reaction advantageously becomes more consistent.
Additionally, the gas collector can include a device for pressing the seal against the lid, and the device can contact both the first member and the second member to press the seal located on the second member against the lid. Still further, the device can include resilient members, which are disposed within the conduit. Also, the device can permit passage of the gasses through the device with an example being springs with open coils. The springs can be positioned in the conduit with seats that are formed in at least one of the members of the body.
In still another embodiment of the present invention, a gas collector includes a body in which is defined a conduit, inlets, an outlet, at least one lip, and a seal. The lip is disposed on a portion of the body adjacent the reaction chamber, and the lip evenly shapes the flow of the gasses into the inlets. Additionally, the lip can be positioned on a portion of the body stationary relative to the reaction chamber. In one aspect, the lip slopes horizontally inwards towards the reaction chamber and slopes vertically away from the inlets. Additionally, the lip can have a curved and/or straight profile.
By providing a lip the shape the flow of the gasses into the inlets, the gasses flow smoother through the reactor. This provides for a more consistent deposition within the reactor. Also, unlike the gas collector of the prior art that includes features that create standing eddy currents, which trap reaction gasses, adjacent the gas collector the lip can be formed to reduce the trapping of reaction gasses. Importantly, trapped reaction gasses could otherwise be reintroduced into the reaction chamber to the detriment of the process, particularly when the process uses multiple gas compositions during the process.
In a further embodiment of the present invention, a gas collector includes a body, in which is defined a conduit, inlets, an outlet, at least one lip, and a seal. The lip supports the body on a platform in the reaction chamber, and the lip completely covers a top surface of the platform. In the gas collector of the prior art, only a portion of the top surface of the platform was covered, and this caused the formation of chips and flakes. However, by completely covering the top surface, the formation of chips and flakes have been reduced.
In still a further embodiment of the present invention, a method for forming deposits on a semiconductor device within a reaction chamber of an epitaxial reactor is disclosed. The method includes introducing reaction gasses into a central portion of the reaction chamber; drawing the gasses radially outward from the central portion into a conduit in a gas collector of the reactor; evenly shaping the flow of the gasses into inlets formed in the gas collector; and forming the deposits on the semiconductor device as the gas flow from the central portion of the reaction chamber into the gas collector. The method can include directing the gasses from the conduit to an exhaust pipe through at least one outlet formed in the gas collector. Additionally, the gasses can be completely removed from reaction chamber before introducing reaction gasses with a different composition into the reaction chamber. Also, the material being deposited can include GaAs.
In yet another embodiment of the present invention, a gas collector includes a body, in which is defined a conduit, inlets, an outlet, and a seal. Also, the body includes graphite. Graphite advantageously remains stable at high temperatures and does not outgas any contaminants or particles. Furthermore, reaction materials such as GaAs readily adhere to graphite, and graphite is a rigid material that resists flexing and has a lower coefficient of thermal expansion as compared to many metals. In addition, graphite has good machinability, which allows for a closer control of dimensional tolerances.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only an exemplary embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.